First this exploit require you to have a group who is okay to help you
Okay, say you've already done an heroic, but you need to do it again ( loot / one try achivements ... ), what you can do is :
1) Queue for random heroic, enter it => It's the one you needed, lucky you !
=> It's not the one you needed then go on reading
2) So you are in an heroic you don't wanna do, normaly you'd have to wait 15 min to retry queuing ... But what you'll do here is queue for the love in the air event.
3) clean the event ( that's 2 minutes + loot with a decent group )
4) Now you can queue again for random heroic !
5) Do this until you queue for the heroic you need, this have personally greatly helped me getting my volcanic drake : )
hope it helps ! ( + sorry, certainly many mistakes cause of my far from perfect english)